The evolution of stars and gas in starburst galaxies

In systems undergoing starbursts the evolution of the young stellar population is expected to drive changes in the emission-line properties. This evolution is usually studied theoretically, with a combination of evolutionary synthesis models for the spectral energy distribution of starbursts and photoionization calculations. In this paper we present a more empirical approach to this issue. We apply empirical population synthesis techniques to samples of starburst and H ii galaxies in order to measure their evolutionary state and correlate the results with their emission-line properties. A couple of useful tools are introduced that greatly facilitate the interpretation of the synthesis: (1) an evolutionary diagram, the axes of which are the strengths of the young, intermediate age and old components of the stellar population mix; and (2) the mean age of stars associated with the starburst, . These tools are tested with grids of theoretical galaxy spectra and found to work very well even when only a small number of observed properties (absorption-line equivalent widths and continuum colours) is used in the synthesis. Starburst nuclei and H ii galaxies are found to lie on a well-defined sequence in the evolutionary diagram. Using the empirically defined mean starburst age in conjunction with emission-line data, we have verified that the equivalent widths of Hβ and [O iii] decrease for increasing . The same evolutionary trend was identified for line ratios indicative of the gas excitation, although no clear trend was identified for metal-rich systems. All these results are in excellent agreement with long-known, but little tested, theoretical expectations.